Method for inhibiting staining of electrodeposited coatings formed in a cyanide containing bath

ABSTRACT

A method for inhibiting the staining of electrodeposited coatings which have been formed in a cyanide containing bath comprising the contacting of the coated objects with an aqueous solution of an aldose sugar.

'1, n 4'" United States v atent 11 1 1111 3,759,192 Gilbert et al. 51 May i, 11973 METHOD FOR INi-HBITING STAINING 3,475,219 10/1969 Laney ..134 27 OF ELECTRODEPOSITED COATINGS 3,505,217 4/1970 Morico ..210/50 FOR ED I A CYA CONTAINING 3,592,774 7/1971 Altenschopfer ..134/2 X BATH FOREIGN PATENTS OR APPLICATIONS 761 t :Lld0.G'lbrt 824E.Ct1 1 men ms Davenpgrte lowa 331 1,184,594 12/1964 Germany ..210/59 S Y I Bennett Watson 909,596 /1962 Great Britain ..21/2.5

9 9 Woods, 63122 OTHER PUBLICATIONS [22] Filed: 1971 F. Pea'rlstein et al., Heat Resistant Chromate Conver- 211 App[ 109 5 sion Coatings, Plating, April 1968, pp. 345-346.

Primary Examiner-Barry Richman [52] US. Cl. ..134/2, 21/25 R, 134/27,

134/29 134/42, 204/ R 210/59, gzggjtyBegarry M. Saragov1tz, Edward J. Kelly and [51] Int. Cl C23b 1/24, C23g 1/26 [58] Field of Search ..l34/2, 42, 26, 27,

134/29; 21 /2.5; 210/59; 204/35 R [57] ABSTRACT A method for inhibiting the staining of elec- [56] Ref Cited trodeposited coatings which have been formed in a cyanide containing bath comprising the contacting of UNITED STATES PATENTS the coated objects with an aqueous solution of an al- 2,648,626 8/1953 Buser ..204/35 R X dose sugar. 2,958,610 11/1960 Ramirez et a1 3,105,822 10/1963 Karabinos et a1 ..134/42 X 7 Claims, No Drawings METHOD FOR INHWHTWG STAWNG 01F ELECTRODEPOSKTED COATFNGS FOD m A CYANIDE CONTG BATH The invention described herein may be manufactured, used, and licensed by or for the Government for govemmentalpurposes without the payment to us of any royalty thereon.

The invention relates to a method of inhibiting the staining of electrodeposited coatings which have been formed in a cyanide containing bath.

It is often desired to coat porous objects such as castings, sintered metals and assemblies having unsealed faying surfaces with copper, zinc, cadmium, gold, silver or other metals plated from a cyanide solution. During the cleaning process prior to and during the electrodeposition, solution enters the crevices, and fills voids in the porous structures. Subsequent to plating and rinsing, remaining hygroscopic solution residues pick up water forming a cyanide solution which bleeds onto the plated surface where it causes corrosion and generally unsightly staining of the metal deposit. This phenomena is known in the trade as spotting out.

While the destruction of the cyanide film may be accomplished using other agents such as an aqueous solution rinse of a halogen, e.g., chlorine, bromine or iodine, these materials in themselves produce undesirable corrosion of the metal surfaces, if retained thereon. In addition, the discharge into the stream of the rinse containing metal halides and metal cyanide residues contribute to water pollution.

We have discovered that the undesired residues of a cyanide containing electroplating bath may be removed by rinsing the objects with an aqueous solution of an aldose sugar, e.g., talose, galactose, idose, gulose, mannose, glucose, altrose and allose. When the cyanide laden parts are introduced into a water solution of, e.g., glucose, the cyanide reacts with the sugar to form sodium heptagluconate, an excellent sequestrant, which chelates the nearby metal ions. This action reduces the corrosivity of the residues and also the toxicity of the waste water. It should be noted that this glucose treatment may be applied prior to plating as well as subsequent thereto. This is particularly applicable where the preplating treatment includes in the conditioning solution, a dilute concentration of sodium cyanide.

It is an object of this invention to provide and disclose a method for inhibiting the staining of electrodeposited coatings which have been formed in a cyanide containing bath.

It is a further object of this invention to provide and disclose an improved method for inhibiting the corrosion of electrodeposited coatings without contributing to the pollution of streams upon the discharge of the effluent rinse material.

Other objects and a fuller understanding of the invention may be ascertained by referring to the following description and claims.

The present invention is illustrated with a description of the method utilized to plate bronze bearings with cadmium to produce a plated surface free from staining. The sintered bronze bearings are first subjected to the vapors of a boiling halocarbon, e.g., trichloroethylene, in a suitable degreaser to remove oils and soil. Treatment in the vapors may be followed with immersion in a vessel of boiling halocarbon solvent with or without applied ultrasonic agitatiomfollowed by a short period of exposure to the halocarbon vapors to produce final rinsing action. Oxides or other tarnishes are then removed by subjecting the sintered bronze to alternate anodic and cathodic polarization in an aqueous solution consisting of 3 pounds of a salt mixture of two parts caustic soda, one part tetrasodium ethylenediamine tetracetate and one part sodium cyanide per gallon of water, maintained at a temperature of up to F. A current density of 50 amperes per square foot is utilized with alternate reversal of the polarity of the work pieces, i.e., 5 seconds positive 10 seconds negative. The bearings are then removed while the polarity is positive, rinsed thoroughly in stagnated water followed by a rinse in a stagnated rinse solution containing 1-5 ounces of an aldose sugar, e.g., glucose, per gallon of water.

The bearings are then transferred to a cadmium plating bath which contains 4.0 ounces of cadmium oxide and 13.1 ounces of sodium cyanide per gallon of solution. The plating bath is maintained at a temperature of around 85 F. A current density (cathode) of 30 amperes per square foot is utilized. A deposit of 0.001 inch thickness is obtained in a period of approximately 20 minutes.

When the desired thickness of deposit has been obtained, the plated parts are rinsed in an aqueous solution of glucose. Concentration of the sugar in the solution may be in an amount of 1 to l 10 ounces per gallon of water and must be maintained so as to provide an excess of sugar beyond that amount stoichiomentically equivalent to the quantities of sodium cyanide being carried into the rinse on the surface and in the pores of the parts being plated. The temperature of this aldose sugar solution may range from below ambient temperature to the boiling point thereof, e.g., 50-212 F. A temperature of F was utilized in the present illustration. Elevated temperatures accelerate the corrosion or staining inhibiting reactions and therefore are favored.

The glucose treated plated objects are then rinsed in cold running water and dried. When extremely porous objects are being plated, repeated alternate immersion in the hot aldose sugar solution and in cold water, with a final rinse in cold water, has been found to be more effective than a single treatment :in the sugar solution followed by a cold water rinse.

Among the many advantages of the present invention are that; no toxic residues are left on the surface of the plated part; no toxic rinse water disposal problems are encountered, and both the aldose sugar and its reaction products with sodium cyanide, i.e., sodium heptagluconate, are totally biodegradable. In addition, both the sugar and the heptagluconate are sequestrants and therefore any residue which may remain on the surface tend to suppress the formation of ionized metallic salts thus eliminating the tendency toward stains and localized spotting out.

Although we have described our invention with a certain degree of particularity, we wish it to be understood that we do not desire to be limited to the exact treatment shown and described, for obvious modifications will occur to a person skilled in the art.

Having described our invention, we claim:

1. A method for inhibiting the staining of electrodeposited coatings selected from the group consisting of copper, zinc, cadmium, gold, silver, and tin, on structures which have been electroplated in a cyanide containing bath comprising the contacting of the coated objects with an aqueous solution of an aldose sugar and thereafter rinsing said objects with water, the concentration of aldose sugar being sufficient to provide an excess of sugar beyond that stoichiomentically equivalent to the quantities of cyanide being carried into the rinse on the surface and in the pores of the objects being coated.

2. A method in accordance with claim 1 wherein the coating is cadmium.

3-. A method in accordance with claim 1 wherein the electrodeposited coatings are subjected to repeated alsugar is glucose.

6. A method in accordance with claim 1 wherein the aqueous sugar solution is maintained at a temperature ofabout 50 to 212 F.

7. A method inaccordance with claim 6 wherein the aqueous sugar solution is maintained at a temperature of around F. 

2. A method in accordance with claim 1 wherein the coating is cadmium.
 3. A method in accordance with claim 1 wherein the electrodeposited coatings are subjected to repeated alternate immersion in a hot aldose sugar solution and rinse water.
 4. A method in accordance with claim 1 wherein the aldose sugar is selected from the group consisting of talose, galactose, idose, gulose, mannose, glucose, altrose and allose.
 5. A method in accordance with claim 4 wherein the sugar is glucose.
 6. A method in accordance with claim 1 wherein the aqueous sugar solution is maintained at a temperature of about 50* to 212* F.
 7. A method in accordance with claim 6 wherein the aqueous sugar solution is maintained at a temperature of around 150* F. 